1. FIELD OF THE INVENTION
The present invention relates to a swash-plate plunger-type hydraulic device such as a swash-plate plunger-type hydraulic pump, a swash-plate plunger-type hydraulic motor, or the like.
2. DESCRIPTION OF THE PRIOR ART:
One known swash-plate plunger-type hydraulic device for use as a pump or a motor is disclosed in Japanese Laid-Open Patent Publication No. 61-118566, for example. Such a swash-plate plunger-type hydraulic device generally has an odd number of plungers that are movable in discharge and suction strokes at different times, or out of phase with each other, for reducing flow rate and torque fluctuations.
Swash-plate plunger-type hydraulic pump and motor may be combined into a hydraulically operated continuously variable transmission. In such a hydraulically operated continuously variable transmission, each of the pump and the motor has an odd number of plungers that are also actuatable in discharge and suction strokes out of phase.
When a plunger shifts in a cylinder from the discharge stroke (compressing stroke) to the suction stroke (expanding stroke), it develops an abrupt change in the hydraulic pressure in the cylinder. The change in the hydraulic pressure is transmitted as vibrating forces to the plunger, the swash plate, and the casing of the hydraulic device. It is known that the transmitted vibrating forces are responsible for the generation of noise from the hydraulic device and the hydraulically operated continuously variable transmission employing the same.
Various attempts have heretofore been proposed to lessen the above change in the hydraulic pressure. For example, pre-compressing and pre-expanding intervals are provided between the discharge and suction strokes, and a restriction passage such as a V-shaped groove, a recess, a regulator valve, or the like is defined to reduce the pressure variation. For details, see Japanese Laid-Open Utility Model Publication No. 63-96372 and Japanese Laid-Open Patent Publication No. 2-129461, for example.
However, the conventional proposals are only effective to attenuate the change in the hydraulic pressure in the cylinder which houses each plunger. The total value of thrust loads imposed on all the plungers is still subject to fluctuations that are applied as vibrating forces. Therefore, it is difficult to lower the noise level to a sufficiently low level.
The fluctuations of the total thrust load will be described below with reference to FIG. 24 of the accompanying drawings. FIG. 24 shows thrust loads F1 through F9 that are applied to respective nine plungers of a swash-plate plunger-type hydraulic pump, and a total thrust load Ft which is the sum of the thrust loads F1 through F9, when the cylinder block rotates. The graph of FIG. 27 has a horizontal axis which is indicative of time, but which may be indicative of the angular displacement of the cylinder block since the angular displacement varies with time. Study of FIG. 27 indicates that the thrust load exerted to each plunger smoothly varies in load increasing and decreasing zones, and the total thrust load Ft fluctuates as shown.
In the case where a swash-plate plunger-type hydraulic pump or motor is of the variable displacement type and has a support shaft by which the swash plate is tiltably supported, or a swash-plate plunger-type hydraulic pump or motor is of the fixed displacement type and has a support shaft similar to the support shaft by which the swash plate is tiltably supported, even if changes in the hydraulic pressure in the cylinder housing each plunger are lessened, variations in the moment about the support shaft, which are also responsible for vibrating forces, cannot sufficiently be suppressed. Therefore, it is difficult to sufficiently lower the noise produced by such a pump or motor.
In such a swash-plate plunger-type hydraulic pump or motor, if it has an even number of plungers, then the pulsating ratio of a discharged flow from the hydraulic device is calculated as follows:
FIG. 25 of the accompanying drawings shows a hydraulic pump model in which a cylinder block 101 has an odd number of angularly spaced cylinder bores 111 defined therein and a number of plungers 112 slidably disposed respectively in the cylinder bores 111, with a swash plate 106 held against the tip ends of the plungers 112. The total stroke L of a plunger 112 is given by: EQU L=2R tan .alpha. (a)
where R is the radius of a circle passing through the centers of the cylinder bores 111, and .alpha. is the angle at which the swash plate 106 is tilted. The displacement D of the plungers 112 is expressed as follows: EQU D=ZAL=2ZAR tan .alpha. (b)
where A is the pressure-bearing surface area of the plungers 112, and Z is the number of the plungers 112.
While a plunger 112 is being angularly moved an angle .theta. from the bottom dead center (BDC), the plunger 112 axially moves a distance x: EQU x=L/2-R cos .theta.tan .alpha.=L/2.times.(1-cos .theta.) (c)
Therefore, the speed v at which the plunger 112 axially moves is given as follows: EQU v=dx/dt=(L.omega./2).times.sin .theta. (d)
where .omega. is the angular velocity of the cylinder block 101.
It is assumed that the number of plungers 112 that are in the discharge stroke is expressed by ZO. From the equation (d), the instantaneous discharge rate Qt of the hydraulic pump is given by: EQU Qt=.SIGMA.Avi=(AL.omega./2).SIGMA.sin .theta.i (e)
The equation (e) can be modified into: EQU Qt=(AL.omega./2).times.sin (.pi.Z0/Z).times.sin {.theta.+.pi.(Z0-1)/Z}/sin (.pi./Z) (f).
Since the number Z of the plungers 112 is even, the equation (f) is therefore modified into: EQU Qt=(AL.omega./2).times.cos (.theta.-.pi./Z)/sin (.pi./Z) (g)
The instantaneous discharge rate Qt is shown in FIG. 26 of the accompanying drawings. As can be understood from FIG. 26, if the number of the plungers 112 is even, then the discharged flow pulsates Z times while the cylinder block 101 makes one revolution. The pulsating ratio .SIGMA. of the instantaneous discharge rate is expressed by: EQU .SIGMA.=.pi./Z.times.tan (.pi./2Z) (h).
According to this equation, actual pulsating ratios .SIGMA. with different numbers of plungers are calculated as follows:
______________________________________ Z 6 8 10 12 .epsilon. (%): 14.0 7.81 4.97 3.45 ______________________________________
The above theoretical study is based on Hydraulic Engineering written by Tsuneo Ichikawa and Akira Hibi.
The foregoing analysis of the pulsating ratio assumes that the hydraulic pressure in the cylinder bores varies according a rectangular pattern as shown in FIG. 27(A) of the accompanying drawings. In actual swash-plate plunger-type hydraulic pumps or motors, however, pre-compressing and pre-expanding zones or restriction passages are employed to cause the hydraulic pressure to vary according to a trapezoidal pattern for thereby preventing the hydraulic pressure from abruptly varying upon a plunger transition from the suction stroke to the discharge stroke and a plunger transition from the discharge stroke to the suction stroke. Consequently, actual pressure changes are indicated by a trapezoidal pattern as shown in FIG. 27(B) of the accompanying drawings. As a result, the actual pulsating ratio differs from the theoretically determined pulsating ratio.
While the trapezoidal pressure pattern is effective in preventing abrupt pressure changes to reduce vibrating forces applied to the swash plate and other components, it rather increases the pulsating ratio, giving rise to abnormal vibration (torque fluctuations), as evidenced by various experiments.